Recently, in order to supply electric power to an electronic device, wireless power supply has begun to come into commonplace use. In order to promote the compatibility of products of different manufacturers, a wireless power consortium (WPC) has been organized, and a Qi standard, which is an international standard, has been developed by the WPC.
A wireless power supply that conforms to the Qi standard uses electromagnetic induction between a transmission coil and a reception coil. A power supply system includes a power supply device including a transmission coil and a power reception terminal including a reception coil.
FIG. 1 is a view illustrating a configuration of a wireless power supply system 10 that conforms to the Qi standard. The power supply system 10 includes a power transmitter (TX) 20 and a power receiver (RX) 30. The power receiver 30 is mounted on an electronic device such as a mobile phone terminal, a smartphone, an audio player, a game machine, or a tablet terminal
The power transmitter 20 includes a transmission coil (primary coil) 22, an inverter circuit 24, a controller 26, and a demodulator 28. The inverter circuit 24 includes an H-bridge circuit (full-bridge circuit) or a half-bridge circuit and applies a driving signal S1, specifically, a pulse signal, to the transmission coil 22 such that a driving current flows through the transmission coil 22, thereby allowing the transmission coil 202 to generate an electrical power signal S2 in the form of an electromagnetic field. The controller 26 performs an overall control of the entire power transmitter 20.
In the Qi standard, a communication protocol is defined between the power transmitter 20 and the power receiver 30, which enables information transmission from the power receiver 30 to the power transmitter 20 via a control signal S3. The control signal S3 in the form of an AM (Amplitude Modulation) modulated signal using backscatter modulation is transmitted from the reception coil 32 (a secondary coil) to the transmission coil 22. The control signal S3 includes, for example, electric power control data (also referred to as a packet) for controlling an amount of electric power to be supplied to the power receiver 30, data indicating unique information of the power receiver 30, or the like. The demodulator 28 demodulates the control signal S3 based on a current or a voltage from the transmission coil 22. The controller 26 controls the inverter circuit 24 based on the power control data included in the demodulated control signal S3.
The power receiver 30 includes a reception coil 32, a rectifying circuit 34, a smoothing capacitor 36, a modulator 38, a load 40, a controller 42, and a power circuit 44. The reception coil 32 receives a power signal S2 from the transmission coil 22, and transmits a control signal S3 to the transmission coil 22. The rectifying circuit 34 and the smoothing capacitor 36 rectifies and smoothes a current S4 induced in the reception coil 32 depending on the power signal S2 to convert the same into a DC voltage VRECT.
The power circuit 44 charges a secondary battery (not shown) using electric power supplied from the power transmitter 20 or steps up or down the DC voltage VRECT to supply the same to the controller 42 and the load 40.
The controller 42 generates an electric power control data (also referred to as a control error (CE) packet) for controlling a power supply amount from the power transmitter 20 such that the rectified voltage VRECT approaches its target value. The modulator 38 modulates the control signal S3 including the electric power control data and modulates a coil current of the reception coil 32, thereby modulating a coil current and a coil voltage of the transmission coil 22.
The Qi standard was initially developed for a low power of 5 W or lower of mobile phone terminals, smartphones, tablet terminals, or the like (Volume I Low Power, hereinafter referred to as Low Power standard). Thereafter, preparation of developing a middle power up to 15 W (Volume II Middle Power, hereinafter referred to as Middle Power standard) is in progress, and the support for a large power of 120 W in the future is planned.
Since the power transmitter 20 and the power reception terminal (electronic device) are disposed in a relatively free space in the power supply system 10, a conductive foreign object such as a metal piece may be placed between or in the vicinity of the transmission coil 22 and the reception coil 32. When wireless power supply is performed in this state, a current may flow through the foreign object, leading to power loss. In addition, the foreign object may generate heat. Thus, in the Qi standard, foreign object detection (FOD) is defined.
In the FOD, power PTX of the power signal S2 transmitted by the power transmitter 20 and power PRX of the power signal S2 received by the power receiver 30 are compared, and, when an inconsistency (difference) between the power PTX and the power PRX exceeds an allowable value, it is determined that a foreign object is present. This is referred to as a power loss method. FIG. 2 is a view illustrating the power loss method. A difference between the transmission power RD(and the reception power PRX is a power loss PLOSS, and the power loss PLOSS becomes greater when a foreign object FO is present and becomes smaller when the foreign object FO is not present. Thus, in the power loss method, it is determined whether the foreign object FO is present or not, based on the power loss PLOSS.
The transmission power PTX and the reception power P are given by the following equations.PTX=PIN−PTX_LOSS PRx=POUT+PRX_LOSS 
Even though PIN and POUT may be precisely measured, it is difficult to accurately measure a power loss PTX_LOSS in the power transmitter 20 and a power loss PRX_LOSS in the power receiver 30. Thus, the measurement values of the transmission power PTX and the reception power PRX include an error to a degree. When an error between the measurement values of the transmission power PTX and the reception power Pp and their actual values is large, an error of the power loss PLOSS may also be increased, causing a problem that the foreign object FO may be erroneously detected or the foreign object FO may not be detected.
In the middle power standard, since the transmission power can be raised up to 15 W, it is severe for a foreign object and more accuracy is required for foreign object detection, as compared with the low power standard. Thus, in the middle power standard, power calibration was defined in order to increase the precision of the power loss method.
FIG. 3 is a view illustrating the power calibration in the middle power standard. The horizontal axis represents a reception power PRX and the vertical axis represents a transmission power PTX. The calibration is performed on the assumption of a situation where a foreign object is not present. A pair of transmission power PTX and reception power PRX are measured at two points P1 and P2. The first point P1 is measured in a light-load state where power supplied from the smoothing capacitor 36 of FIG. 2 to a load (not shown) is sufficiently small. In the light-load state, the power receiver 30 generates a CE packet such that a rectified voltage VRECT is identical to a target value. In response to the CE packet, the power transmitter 20 transmits a required transmission power PTX. The power receiver 30 measures an output power POUT and, further, a reception power PRX_LIGHT, based on the product of the rectified voltage VRECT and a load current, and transmits a control signal S3 indicating the measured output power POUT and reception power PRX_LIGHT to the power transmitter 20. When the power transmitter 20 receives data indicating the reception power PRX_LIGHT, it returns an acknowledgement (ACK) to the power receiver 30. Further, the power transmitter 20 measures a transmission power PTX_LIGHT in the light-load state.
The second point P2 is measured in a state (referred to as a connected state) where a load current from the smoothing capacitor 36 of FIG. 1 to the power circuit 44 is increased. Also, in the connected state, the power receiver 30 generates a CE packet such that the rectified voltage VRECT is identical to a target value. In response to the CE packet, the power transmitter 20 transmits a required transmission power PTX. The power receiver 30 measures a reception power PRX_CONNECTED based on the product of the rectified voltage VRECT and a load current, and transmits a control signal S3 indicating the measured reception power PRX_CONNECTED to the power transmitter 20. When the power transmitter 20 receives data indicating a reception power PRX_CONNECTED, it returns an ACK to the power receiver 30. Further, the power transmitter 20 measures a transmission power PRX_CONNECTED in the connected state.
As aforementioned, calibration may need to be performed in a state where a foreign object is not present, and when the calibration time is lengthened, there is a high possibility that the foreign object interferes with the calibration. Thus, while the Qi standard requires that the calibration should be completed within a short time such as about 10 seconds, the power measurement at the two points in the light-load state and the connected state may not be completed within such a short time.